Head bolts snapping

[spock500]

Hi all,

The repair of my C250D continues to be an enigma to me

Today I went to fit the reconditioned head with a new gasket set, all was fine until I wen to torque down the bolts -

I progressively went in sequence tightening the bolts 4/5 times with the (new) torque wrench set at 5ft lbs the lowest it will go, (Haynes recommend 11ft lbs for stage one) I kept tightening but nothing, the wrench didnt 'click' instead one of the bolts snapped

Picture below is of the bolt and wrench (setting has moved at bit but it was on 1) The bolts are not new as they are within 1mm of the stretch tolerances.

 

[BTB 500]

with the (new) torque wrench set at 5ft lbs

With a new torque wrench (or one that's not been used for a while) I think you're generally supposed to 'exercise' first it to make sure the mechanism is working freely. Is that not mentioned in the instructions?

 

[spock500]

With a new torque wrench (or one that's not been used for a while) I think you're generally supposed to 'exercise' first it to make sure the mechanism is working freely. Is that not mentioned in the instructions?

Nope, its brand new one of these

http://cgi.ebay.co.uk/3-8-drive-tor...ryZ30906QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

 

[BlackC55]

Did you clean out the bolt hole?

 

[television]

I think that the torque wrench is faulty 10ilb is equal to a 6" spanner

 

[grober]

Did the torque wrench come with any calibration certificate. Could be a hydraulic lock like Black C55 has said.

 

[BTB 500]

Try the torque wrench on a wheel nut or something. As Malcolm says, it could be faulty.

 

[tlzeebub]

At 5ftlb the click will almost be unoticable, but to shear them bolts like that must have taken a lot of force, how much effort were you putting into the wrench when they snapped?

 

[BTB 500]

The hole being blocked would just mean the bolt wouldn't be fully home when it reached the specified torque. To shear the bolt off either the bolt or the wrench is faulty IMO.

 

[spock500]

Yup, I cleaned the holes and then generously gave a squirt of oil in the hole and on the bolt head/thread -

Dont tell me the hole is shallow and by filling with oil its locked it, never thought of that but the wrench should have clicked out -

 

[jaymanek]

To shear the bolt off either the bolt or the wrench is faulty IMO.

I know where im placing my bet looking at how much it costs..

 

[spock500]

At 5ftlb the click will almost be unoticable, but to shear them bolts like that must have taken a lot of force, how much effort were you putting into the wrench when they snapped?

Both arms and made quite a 'crack' when it went, your limited to the force your able to apply because of the extension required to be over the lip -

I have loosened all the bolts off in reverse sequence, I wanted to try the wrench out on a wheel nut but dont have a 3/8 to 1/2 reducer -

Can anyone rec a good wrench, the ones on ebay are very similar twisting handle type, this looks ok
http://www.halfords.com/webapp/wcs/...1_productId_178843_langId_-1_categoryId_76866

 

[Rory]

Even expensive torque wrenches are not accurate at the extremes of their range.

You could test it pretty easily using a wheel and some weights - see how much effort it takes to lift the weights.

I think you've probably just missed the click though.

 

[grober]

The Halfords torque wrenches got good reviews here http://www.autoexpress.co.uk/products/products/59647/torque_wrenches.html

 

[television]

Never put oil down a blind hole,, it can crack the casting

 

[spock500]

The Halfords torque wrenches got good reviews here http://www.autoexpress.co.uk/products/products/59647/torque_wrenches.html

Only problem is it starts at 30 ft lbs which is quite high i guess, the first stage on my head sequence is 11

 

[tlzeebub]

Both arms and made quite a 'crack' when it went, your limited to the force your able to apply because of the extension required to be over the lip -

I have loosened all the bolts off in reverse sequence, I wanted to try the wrench out on a wheel nut but dont have a 3/8 to 1/2 reducer -

Can anyone rec a good wrench, the ones on ebay are very similar twisting handle type, this looks ok
http://www.halfords.com/webapp/wcs/...1_productId_178843_langId_-1_categoryId_76866

Blimey 5ftlb's you would be able to do with one hand, if you did a screw up with a medium sized srewdriver you would probably be going to about 5ftlb's, so i think you didnt feel the wrench click

 

[spock500]

Ok, problem solved with the wrench -

I put the socket into a fixed mount then set it to 10ft, nothing using max force - then twisted the grip all the way to the top (80 ft) and back half way tried again and 'click', went down to 10 ft and 'click' (very easy)

So I guess it was jammed or in transit mode :bannana:

Thanks all for your help

 

[Number_Cruncher]

The way that the bolt has necked down in the OP's picture suggests a failure in tension. That the click was absent, or missed seems a fair conclusion.

However, it's likely that all the other bolts have been damaged, and to be safe should at least be checked to make sure their length is still within tolerance, or simply renewed.

When a bolt has a sequence of torque followed by a series of angles, the first torque is just a snug torque - if you get it a bit wrong, it will make no significant difference. So, I wouldn't worry too much about using 20 lb ft as the snug torque rather than 11 if that will make your torque wrench click more noticably.

Torque wrenches - even if perfectly calibrated and accurate are a very poor way to acheive an accurate bolt pre-tension. Pre-tension is what you are trying to achieve when you tighten a bolt - the torque is just an intermediate step.

Torque wrenches are a wildly inaccurate method of applying pre-tension to a fastener, because there are friction processes, both in the threads themselves, and more importantly under the head of the bolt which affect the relationship between torque and pre-load. It is to side-step this source of error that the angle technique is used.

Even done sloppily, the angle method gives much more repeatability between the pre-tension of bolts than does bolt installation using just a torque wrench. It's not unusual to have 30% variation among the pre-load of a set of nominally identical bolts tensioned using a torque wrench. You would have to be very sloppy to be 30% out using an angle method.

The other way to look at this is that if a fastener on a car has just a torque value, then it probably isn't too sensitive to being torqued up to that figure. If the joint were sensitive to preload a better tightening specification would have been given.

 

[spock500]

Even done sloppily, the angle method gives much more repeatability between the pre-tension of bolts than does bolt installation using just a torque wrench. It's not unusual to have 30% variation among the pre-load of a set of nominally identical bolts tensioned using a torque wrench. You would have to be very sloppy to be 30% out using an angle method.

Thanks for the reply,

This is what the haynes manual states:

Stage 1, 15nm or 11 lbf ft
Stage 2, 35nm or 26 lbf ft
Stage 3, Angle tighten through a further 90 degree
Stage 4, wait for 10 mins
Stage 5, Angle tighten through a further 90 degree (90o)

Does stage 2 seem tight enough, a couple of posters have suggested a lot higher for ally heads?

What precisley is 'angle tighten' ?